The present invention relates to a measuring weir for liquids. More particularly this invention concerns such a weir used in rivers, channels, sewers and at combined sewer overflows (CSO) for determining the flow rate of water flowing over it.
Such a weir is used to determine the actual flow rate in l/s or the flow capacity in m3/a that passes over itself. It is furthermore known that well established overflow weirs work only with a limited overflow range. There are however circumstances when the overflow for a long time is rather small and then can briefly have a great volume. A wide range of flowrates in a length- and short-term mode should be correctly determined.
With the known overflow weirs there is no satisfactory correlation between the overflow rate and the upstream water level above the upper edge of the overflow weir. In order to measure the overflow amount sharp-edge weirs have been suggested (ATV A 111, ISO 1438). These are identical to the measuring weirs of Rehbock. With such sharp-edged weirs there is with small drops of less than 30 mm an uncontrolled adhesion or release of the overflow stream from the rear side of the weir and thus there is strong hysteresis in the discharge rating curve. The often proposed artificial aeration of the underside of the overflowing stream is not effective and not practicable. With overflow heads of less than 10 mm there are further problems created by water surface tension.
A sharp-edged weir has with substantial overflow heads a high resistance to flow expressed as a small xcexc value in the overflow formula of Poleni. Small xcexc values produce large heads. Backwater is created in the upstream canal network.
To control flow in rivers or over dams, broad crested weirs with rounded tops are known. Such rounding is employed in order to allow substantial volumes of water to pass with minimal head losses and noise. Such weirs are however not usable in waste-water systems due to their limited measuring range.
In order to improve measurements a top profile for an overflow measuring weir is known that is made of a combination of a standard profile top and a horizontally extending downstream portion projecting from the rear side of the weir. The standard profile top is relatively greatly curved and forms a nearly straight rear face with an angle of about 45xc2x0 merging over a concave transition with a nearly horizontal rear surface (EP 0,477,745). 
The problem of getting accurate measurements is furthermore made harder by the normal use upstream of the overflow weir of a scum board or baffle which serves to prevent flotsam and light liquids from escaping out of the waste-water system. With high flow volumes such baffles substantially disrupt the water flow.
It is therefore an object of the present invention to provide an improved flow-measuring weir.
Another object is the provision of such an improved flow-measuring weir which overcomes the above-given disadvantages, that is which can produce accurately reproducible and free of hysteresis measurements over a wide range of flows with relatively little flow resistance with great overflow rates and high resistance at low flow rates.
These objects are attained in a flow-measuring system having a weir wall bounded on a front face by a body of water and having a rear face and a top edge over which water from the body flows that is provided with structure generally formed as an airfoil on the top edge positioned such that water flowing over the weir wall must flow over the structure.
Such a weir-wall structure designed according to aerodynamic principles does not actually create lift on its upper side, but ensures that the water flowing over it remains on its upper side and exits as a smooth sheet from the rear edge. Thus even without aeration, the structure is free of hysteresis.
According to the invention the structure is mounted on the weir wall and is a separate element. In a standard application, the wall itself is made of reinforced concrete and the airfoil-shaped structure is installed on it after construction. This makes it possible to provide a perfectly shaped structure on top of a wall that is built to less strict tolerances.
In accordance with the invention the structure has a forward high point and a rear edge and is smoothly and gently upwardly convex between the high point and the rear edge. It has no concave turbulence-creating portions that could cause lift-off or even cavitation at high flow speeds. In addition the rear edge is horizontally rearwardly directed and sharp. Thus even with a low flow speed or a small drop, the water will separate easily from the lower edge of the structure at a defined and given point as a smooth curtain that will drop generally free of the rear wall face that is spaced well forward of this rear edge.
The structure extends downward according to the invention at an angle between 10xc2x0 and 15xc2x0 from the front high point to the rear edge. This convexity ensures that the water stays in contact with the upper structure surface. The gentle rounding is completely barrier free. Thus even with very light flow of 1 l/(s m) the flow regime is dominated by shear stress and boundary layer effects and it is possible to measure reproducible xcexc values. This is a huge advantage when measuring modest flows.
With larger flows the airfoil shape is very advantageous so that a large volume of water can flow over it with small losses. Gravity flow is dominating this regime. Once a certain minimal flow over the rear edge is ensured, the flow volume can grow to virtually any size with no difference in the capacity to measure it.
In addition in accordance with the invention the forward high point is generally level with the front face of the wall, normally directly aligned with it. The structure is formed forward of the high point with a horizontally forwardly directed smooth bulge. It is made of smooth stainless steel and is vertically adjustable on the wall, for instance by securing it in place with screws extending through vertical slots. It is also of horizontally adjustable width, something made possible by making it of two parts, one fitted into a horizontal slot of the other. What is more the structure can be made of a succession of pieces spaced along the wall and interfitting with slip joints to compensate for thermal expansion.
The structure is associated with an upstream baffle or scum board having a lower edge in the body of water forward of the front face. This baffle lower edge is bent forward away from the wall. Furthermore a sensor is provided forward of the baffle for measuring a level of an upper surface of the body of water, preferably one that works by ultrasonic reflection or the like so that it does not actually have to touch the water.